The three-dimensional structure of a variety of dehydrogenases is being investigated by means of X-ray crystallography. The structure of lactate dehydrogenase (LDH) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) have been determined. At this time complexes of these structures of these enzyymes related to different periods during catalysis are being investigated and compared. Thus a careful comparison of a ternary NADH:oxamate:LDH complex with apo-LDH is underway, as well as a determinaton of the apo-GAPDH structure in relationship to the previously investigated holo-GAPDH molecule. A variety of LDH isozymes (M4, H4, and X) are also being studied in order to understand their different physical and chemical properties. A study of yeast LDH is in its very first stages. The latter is a rather different enzyme being FMN and heme linked. It is hoped that this will lead to a further understanding of the evolution of structure and function of proteins.